1. Technical Field
This invention relates generally to electric motors and more particularly to two-dimensional electric motors.
2. Background Art
Electric motors are used in a variety of electrical equipment. For example, linear electric motors produce electrical power propelling an armature in one dimension. Wafer stages use linear electric motors to position a wafer during photolithography and other semiconductor processing.
A typical one-dimensional linear electric motor has a magnet track with pairs of opposing magnets facing each other. Within spaces between the pairs of opposing magnets, an armature moves. The armature has windings of a conductor which are connected to an electrical current. When the electrical current is turned on, the electric current interacts with the magnetic fields of the magnet pairs to exert force on the armature, causing the armature to move. When the armature is attached to a wafer stage, the wafer stage experiences the same force as and moves in concert with the armature.
In a multiphase motor, the armature has various windings grouped into phases. The electric currents are selectively applied to the phase groups to create a more efficient motor. As the armature moves within the magnet track as current is applied to a first group of coils, the first group moves out of its optimal position between the pairs of magnets. Then, it becomes more efficient to apply current to a second group of windings. More phase groups are theoretically more efficient since a more even application of force and use of power input is maintained. However, each additional phase group complicates timing of the applied current to the various phase groups. Presently, three-phase motors and armatures have gained favor in balancing these considerations.
Linear two-dimensional motors also are used in manufacturing. (U.S. Pat. No. 4,654,571, entitled “Single Plane Orthogonally Moveable Drive System,” issued to Hinds on Mar. 31, 1987 (“Hinds”) and U.S. Pat. No. 4,535,278, entitled “Two-Dimensional Precise Positioning Device for Use in a Semiconductor Manufacturing Apparatus,” issued to Asakawa on Aug. 13, 1985 discuss two-dimensional linear electric motors and are incorporated herein by reference in their entireties.) The motors are two-dimensional in that they have two-dimensional arrays of magnets and armatures instead of magnet tracks and one-dimensional armatures. However, the magnet arrays and two-dimensional armatures may move with respect to each other in more than two dimensions depending upon the design. Conventional two-dimensional linear motors typically have an array of magnets and an armature having one or more coils on one side of the array of magnets.
U.S. Pat. No. 5,623,853, entitled “Precision Motion Stage with Single Guide Beam and Follower Stage,” issued to Novak et al. on Apr. 29, 1997 and U.S. Pat. No. 5,528,118, entitled “Guideless Stage with Isolated Reaction Stage,” issued to Lee on Jun. 18, 1996 discuss examples of semiconductor fabrication equipment and are incorporated herein by reference in their entireties.
When attached to part of a two-dimensional linear motor, a platform can be moved in two or more dimensions by the motor. For example, a wafer stage in semiconductor processing equipment may be attached to an armature or magnet array of a two-dimensional motor and the two-dimensional motor would control positioning of the wafer stage.
When used to position a platform, conventional two-dimensional electric motors do not smoothly and accurately position the platform. Presently, coils in the two-dimensional electric motors move with respect to the magnets. As exemplified in U.S. Pat. No. 4,654,571, entitled “Single Plane Orthogonally Moveable Drive System” issued to Hinds on Mar. 31, 1987, referenced above and incorporated herein by reference in its entirety, cables and hoses are attached to the coil assembly. The cables are for electrical current and the hoses may be used to carry coil cooling fluid or air supply. Unfortunately, the hoses and cables impede free motion of the coil assembly. If the hoses could be eliminated, the stability of motion of the motor and positioning of the platform would be improved.
Also, in many cases conventional technology relies upon cumbersome stacked arrangements to achieve six degrees of freedom movement of the platform. The six degrees of freedom include three translational and three rotational degrees of freedom. (Richard P. Feynman, Robert B. Leighton, and Matthew Sands, The Feynman Lectures on Physics, Addison-Wesley, 1962, discusses translational and rotational motion and degrees of freedom and is incorporated herein by reference in its entirety.) Unfortunately, many designs obtain six degrees of freedom by essentially stacking multiple two dimensional and/or one dimensional motors which move only in two dimensions within a plane. (U.S. Pat. No. 5,623,853, entitled “Precision Motion Stage with Single Guide Beam and Follower Stage” issued to Novak et al. on Apr. 29, 1997, discusses examples of such stacked arrangements and is incorporated herein by reference in its entirety.) For example, a platform may be propelled back and forth in one dimension under the control of linear electric motors. The linear electric motors are part of a holder which holds the platform. In turn, a second holder holds that entire holder and platform arrangement via joint connections and propels it back and forth in a second dimension by another set of linear electric motors. Additional degrees of motion may be provided by voice coil motors which are attached to these holders.
These types of stacked arrangements have a few drawbacks. Each additional holder enabling more degrees of freedom also adds mass requiring additional power for the electric motors to move the platform. Also, the complicated joint connections degrade accuracy of positioning of the platform and create additional resonant vibration frequencies.
Recognizing that the platforms could benefit from a better electric motor to position them, such as an improved electric motor that would eliminate the air hoses and position the platform in multiple degrees of freedom without the cumbersome stacked arrangements, U.S. Pat. No. 6,208,045 issued Mar. 27, 2001 to Hazelton et al. provided a basic planar motor. Planar stage motors generally provide a wafer stage directly on a base, with the motor acting directly on the stage. Another example of a planar motor is that of U.S. Pat. No. 4,654,571, mentioned above (Hinds), that uses a single motor to control motion in three or six degrees of freedom. U.S. Pat. Nos. 5,196,745 and 3,851,196 exemplify planar stage motors, particularly, planar stage motors that use multiple linear motors to achieve motion in three or six degrees of freedom. U.S. Pat. No. 5,334,892 exemplifies a planar motor that can use either a checkerboard magnet array, or separate linear motor magnet arrays.
Compared to stacked designs, planar stage motor designs eliminate mass, such as eliminating guide bar mass and “y” linear motor mass. However, planar stage motors could be improved and made more sophisticated, such as by better incorporating the advantages of stacked arrangements while avoiding re-introduction of the disadvantages of stacked designs.
In summary, the conventional planar motors using a single actuator to produce forces in both the x and y directions, typically using a checkerboard magnet array, have several drawbacks. First, at most 50% of the armature area can be used to generate force in the x or y direction (assuming the motor's performance in x and y are equal), because the remaining area is needed for the other axis. Second, these single actuator designs often result in coupling between forces in different directions. For example, in the motor in U.S. Pat. No. 6,208,045, each coil produces force in the x, y and z directions. Because of the different forces produced by multiple coils, proper commutation permits independent control of force in each direction. For example, to produce x force only, the currents in a group of coils can be commutated so that the y and z force components cancel each other. However, because of manufacturing variations, changes in the magnetic gap, and other disturbances, the forces produced by each coil do not exactly match theory. Therefore, producing force in the x direction will lead to some force ripple in y and z. Also, the effects of cross-axis force ripple limit the planar motor's ability to precisely control the position of the stage. As a result, lithography machines that use these designs would require a stage that uses the planar motor for coarse positioning, and carries a high-precision fine-stage for precise position control.
Conventional linear motors have not resolved these problems of single actuator motor designs. Designs that use distinct linear motors, such as those of U.S. Pat. Nos. 5,196,745 and 3,851,196, have a limited range of motion. Linear motors that address this problem with limited range of motion have the drawback of requiring motor structure on both sides of the stage. Often the parts of the motor on the top of the stage interfere with other sensors or equipment. Also, such a design requires a relatively large stage. These various disadvantages of conventional linear motors detract from their efficiency, performance and usefulness.